The present invention relates to a screen apparatus that includes a microlens array and provides high transmission and high image contrast, as well as rejection of ambient light. The three fundamental components of the screen constitute a microlens array, e.g., a random microlens array, a substrate that supports the array, and a layer of light-absorbing material. The substrate and the microlens array can be separate components or a single unitary component.
The microlens array is preferably composed of generally anamorphic lens units that scatter and shape light according to distinct divergence angles along perpendicular directions. The use of random microlens units is also preferred since it eliminates the occurrence of image artifacts such as aliasing or moirxc3xa9 patterns. In addition, the size of each microlens unit in the array is preferably chosen small enough to provide high resolution. A typical microlens diameter or, more generally, maximum transverse dimension is between 20 and 120 microns.
The substrate, generally of a plastic material, provides support and mechanical rigidity to the whole screen. In the formation of the screen, an absorptive layer is added to the substrate, opposite to the microlens side, to provide ambient-light rejection and improve image contrast. Initially the light-absorbing material is added as a film layer, which is later exposed with aperture-forming illumination, e.g., ultraviolet illumination, to create apertures through which light propagates. The microlens array itself serves as the focusing element that creates the apertures.
A major contribution to the art of the present invention relates to the design of the microlens units to attain efficient focusing of the aperture-forming illumination throughout the array. Each lens element in the array is thus optimized to present a focus plane close to the absorptive layer so as to maximize the density of absorptive material while allowing the image-forming illumination to reach the viewer substantially unimpeded. The present invention can be used for general screen applications such as in rear-view projection televisions, computer screens, and general-purpose displays, among other devices.
Display devices consist, in a broad sense, of an illumination system that provides luminous power (the light engine), relay optics such as lenses and mirrors, and a screen that projects information through images to the viewer in the so-called viewing space. The present invention relates to the screen component of display devices.
Desirable optical qualities for screens include high transmission, high contrast, rejection of ambient light, absence of image artifacts, high gain, and wide viewing angles. The basic elements of the screen responsible for these optical properties include a diffusing component, a supporting substrate, and an absorptive material. The diffusing component spreads the illumination in a controlled manner to direct the visual information to locations most likely occupied by the viewer. The absorptive material minimizes reflection of ambient light that reduces image contrast. Important examples of display devices include liquid crystal projection TV""s; CRT projectors that use three distinct color sources in the light engine; flat-panel computer displays; and hand-held computing devices.
As the demand for high-quality displays increases, so does the necessity for adequate screen designs. For instance, the advent of HDTV (high-definition television) requires a considerable increase in image resolution, which implies that the screen must be able to resolve very fine features in the images being projected. To minimize power consumption and maximize brightness the screen must transmit a high fraction of the luminous power generated by the light engine. It is desirable that the transmission efficiency exceed 80%. On the other hand, to provide image contrast, the screen requires some sort of absorptive material that helps reject ambient light. If not properly designed, this absorptive material may lead to a significant decrease in transmission efficiency.
The diffusing component of screens used in virtually all commercial display systems to date rely on the use of lenticular arrays and/or random Gaussian surface diffusers. The lenticular arrays consist of cylindrical lenses with pitch between 300 microns and up to 700 microns. An example of this type of screen can be found in U.S. Pat. No. 5,870,224. More recently, improved lenticular screens with pitch down to 150 microns have been used. Because cylindrical lenses diffuse the illumination in a single direction (usually the horizontal) it must be combined with another diffusing component in the perpendicular direction, generally a random surface diffuser, if some wider viewing range is desired in the perpendicular direction.
The lenticular array and random Gaussian surface diffusers, while commercially available, present inherent disadvantages as follows. The lenticular array, because of its periodicity, may lead to diffraction effects as well as moirxc3xa9 fringing effects. Furthermore, lenticular arrays provide limited control over the distribution and shaping of light in the viewer space. Gaussian surface diffusers have the serious disadvantage of introducing speckle, which adds a grainy appearance to the image, unacceptable to the viewer. Also, Gaussian diffusers offer limited control over the scattering pattern. U.S. Pat. No. 4,666,248 discloses a screen geometry based on the use of a regular array of anamorphic lenses, in an attempt to obtain more control over the scattering profile. The regular nature of the array, however, does not avoid problems with diffraction and moirxc3xa9 fringing effects. Furthermore, the size of each microlens unit is between 300 and 500 microns, offering insufficient resolution for the increasingly high demands of visual display systems.
To provide a screen with ambient-light rejection capabilities and improved contrast it is necessary to introduce a light-absorbing component to the screen. A great majority of screens in commercial use add a bulk absorbing material (tint) to the body of the screen that, while adding contrast to it, also consumes a considerable fraction of the useful illumination originating in the light engine. In fact, transmission efficiency is typically below 60% and in many cases even less.
A more attractive approach relies on adding a layer of light-absorbing material to a screen that has focusing elements and has the absorbing material perforated in such a way that light is focused through the apertures, as illustrated in FIG. 1. This is the approach described in U.S. Pat. Nos. 5,870,224; 4,666,248; 5,066,099; and 4,721,361. The main advantage of this scheme is that high transmission may be maintained even if the screen presents a high density of light-absorbing material.
Although this approach seems promising, there are some known difficulties such as quality of the apertures, mask alignment for aperture formation, and uniformity of the dark absorptive material. Also, the focusing array in the prior art has never been optimized to operate with the light-absorbing material, except for the use of a common focal distance for all focusing units. As a result, demonstration of the concept of a high-transmission screen through apertures perforated on a light-absorbing material has not been satisfactorily achieved by the prior art and is unavailable in current commercial display systems.
Some screens commercially available use focusing elements such as glass beads immersed on a light-absorbing material but this approach consumes a large fraction of the incident illumination and, therefore, cannot be considered satisfactory.
As a further aid in ambient-light rejection, U.S. Pat. Nos. 4,666,248 and 4,721,361 disclose a concept where a surface of the screen is structured with anti-reflection capabilities but this adds further complexity and may lead to image artifacts, unacceptable to the viewer. Furthermore, if the density of light-absorbing material in the screen is high, such as above 70%, there is little need for the further complication caused by an anti-reflection structured surface.
In contrast to the foregoing, the present invention is geared towards high density of light-absorbing material and thus allows a simpler screen architecture.
The present invention addresses the drawbacks of the prior art, as described above, providing a novel and improved screen configuration that offers high transmission, high contrast, efficient ambient light rejection, high resolution, absence of image artifacts due to diffraction and moirxc3xa9 fringing, and widely controllable scattering angles to the viewer.
In accordance with one of its aspects, the invention provides a screen configuration comprising a diffusing element in the form of an array of microlenses which are preferably both random and generally anamorphic, a substrate, and a layer of light-absorbing material. In a first step, the microlens array, the substrate, and the layer of light-absorbing material are integrated in a single sheet. (As discussed above, the microlens array and the substrate can constitute a single unit if desired.) In a second step, the sheet is exposed to aperture-forming illumination, e.g., ultraviolet light, through the microlens array itself to create apertures in the light-absorbing material. There is no need to use external alignment masks and the initial self-alignment guarantees that light is efficiently transmitted through the apertures. The microlens array is especially designed such that the apertures created by the aperture-forming illumination preferably do not block any portion of the useful luminous energy from the light engine (the image-forming illumination). At the same time, the microlens array is especially designed to maximize the density of light-absorbing material remaining after the creation of the apertures. By apertures we mean either physical apertures (holes) or a transparent area in the light-absorbing material. The particular case depends on the interaction between the light-absorbing material and the aperture-forming illumination, e.g., the interaction can constitute ablation or a photo-chemical reaction.
In accordance with another aspect, the invention provides a random microlens array that focuses substantially all the aperture-forming incident light onto a light absorbing layer to create apertures through which image-forming illuminating radiation propagates. The light-absorbing material is located with respect to the microlens array so that the size of each aperture is minimized while at the same time allowing the screen to produce an acceptable image.
In accordance with a further aspect, the invention provides an array of anamorphic microlenses (preferably, a random array) such that light is focused at two distinct spatially-separated planes and creates distinct viewing angles away from the screen. Each microlens element is optimized so that the first focal plane away from the microlens array is substantially aimed at the light absorbing layer to create apertures through which image-forming illuminating radiation propagates. The light-absorbing material is located with respect to the microlens array so that the size of each aperture is minimized while at the same time allowing the screen to produce an acceptable image.
In accordance with a still further aspect, the invention provides an array of anamorphic microlenses (preferably, a random array) such that light is focused at two distinct spatially-separated planes and creates distinct viewing angles away from the screen. Each microlens element is optimized so that the second focal plane away from the microlens array is substantially aimed at the light absorbing layer to create apertures through which image-forming illuminating radiation propagates. The light-absorbing material is located with respect to the microlens array so that the size of each aperture is minimized while at the same time allowing the screen to produce an acceptable image.
In accordance with an additional aspect, the invention provides an array of anamorphic microlenses (preferably, a random array) where the diameter of each microlens unit is different along two perpendicular directions so that the spatially-separated focal points of the micro lenses are brought closer together, while maintaining distinct divergence angles for two perpendicular directions. The light-absorbing material is located with respect to the micro lens array so that the size of each aperture is minimized while at the same time allowing the screen to produce an acceptable image.
The invention also provides a method that allows the depth of the microlens array and the thickness of the substrate to be chosen arbitrarily and independently of each other in the case where the desired divergence angles differ along two perpendicular directions.
In a realization (embodiment) of the invention, the microlens units in the array are arranged in a close-packed square array.
In a realization (embodiment) of the invention, the microlens units in the array are arranged in a close-packed rectangular array.
In a further realization (embodiment) of the invention, the microlens units in the array are arranged in a close-packed hexagonal array.
In another realization (embodiment) of the invention, the microlens units in the array have spherical boundaries and are arranged in a hexagonal array.
In a further realization (embodiment) of the invention, the spatial arrangement of the microlenses is random with each microlens unit defined by general polygonal boundaries.
According to a realization (embodiment) of the invention, the apertures formed in the light-absorbing material have the shape of horizontally-modulated lines.
In another realization (embodiment) of the invention, the apertures formed in the light-absorbing material have the shape of vertically-modulated lines.
According to a realization (embodiment) of the invention, the apertures formed in the light-absorbing material have the shape of horizontal ovals in a hexagonal spatial arrangement.
According to a realization (embodiment) of the invention, the apertures formed in the light-absorbing material have the shape of horizontal ovals in a square spatial arrangement.
According to a realization (embodiment) of the invention, the apertures formed in the light-absorbing material have the shape of vertical ovals in a hexagonal spatial arrangement.
According to a realization (embodiment) of the invention, the apertures formed in the light-absorbing material have the shape of vertical ovals in a square spatial arrangement.
According to a realization (embodiment) of the invention, the apertures formed in the light-absorbing material have the shape of horizontal ovals of varying size with a seemingly random spatial arrangement.
According to a realization (embodiment) of the invention, the apertures formed in the light-absorbing material have the shape of vertical ovals of varying size with a seemingly random spatial arrangement.
As used herein, the term xe2x80x9canamorphicxe2x80x9d and the phrase xe2x80x9cgenerally anamorphicxe2x80x9d refers to a lens (e.g., a microlens) which has different optical powers along two orthogonal axes, the difference between the two powers being greater than 5 percent, i.e., "PHgr"1xe2x88x92"PHgr"2/"PHgr"1 greater than 0.05 where "PHgr"1 greater than "PHgr"2.